The generation and decay of transient species in light-scattering materials is examined theoretically. Two limiting cases of transient distribution can be identified for which straightforward mathematical solutions are available: ( a ) where an exponentially falling-off concentration of transients exists beneath the sample surface and ( b ) where a homogeneous transient concentration (a 'plug') exists beneath the sample surface. Between and including these limits, the concentration profile can be calculated by a numerical iterative thin-slice approach. This method also permits calculation of the experimentally available reflectances R of the sample at the analysing wavelength. The size of the transient absorption is governed by the extinction coefficients of the initial absorber and the transient. In the case of a transient species which shows both luminescence and absorption, it is found that the emission intensity and the transient absorption at the same wavelength are proportional to each other. For a transient decaying by a unimolecular pathway, the rate constant can be obtained in case ( a ) from the slope of a plot of In (1 -R ) us. time, while for case ( b ) In [ F ( R ) ] must be plotted against time, where F ( R ) is the Kubelka-Munk function. If the decay follows a bimolecular pathway, the rate constant can be evaluated from the slope of a l / F ( R ) plot us. time only for case ( b ) . In all other cases no straightforward method is available. Experimental data are presented and discussed for benzil microcrystals, an example of case ( a ) where the predicted superimposition of transient decay and phosphorescence is also observed, and for acridine adsorbed on silica gel, an example of case ( b ) .

The nature and mobility of the species present when acridine is adsorbed from high vacuum onto thermally pretreated alumina and silica have been investigated by ground-and excited-state spectroscopic techniques. For alumina treated at high temperatures (>500 °C) -bonded acridine is the main species present, while the acridinium cation AH+ predominates at lower activation temperatures (-100 °C). The triplet-triplet absorption at 480 nm of AH+ has been directly observed without a triplet sensitizer, and the decay is nonexponential. On silica the main adsorbed species is hydrogen-bonded acridine which shows triplet-triplet absorption at 435 nm and which, for samples of low coverage or high pretreatment temperature, has an exponential decay with a lifetime of about 30 ms. For samples of high coverage or low pretreatment temperature, the triplet decay is faster and nonexponential, and delayed fluorescence is observed. This arises from triplet-triplet annihilation, a consequence of the mobility of acridine on the silica surface for which the two-dimensional bimolecular rate constant is 8 x 1013 dm2 mol"1 s"1. No delayed fluorescence occurs when silica has been pre.rented at high temperatures, conditions which favor dehydroxylation of the surface to form siloxane units. These have a high activation barrier to translational motion and thereby impede the lateral mobility of the adsorbate. For alumina, the nature of the surface permits virtually no mobility of the acridine, and hence no significant bimolecular processes are observed on this adsorbent.

. J. Chem. 68, 812 (1990).Diphenylmethyl radicals have been generated by 266 nm laser excitation of 1,1,3,3-tetraphenylacetone adsorbed on silica gel and included in NaX and Silicalite zeolites and have been studied using diffuse reflectance laser flash photolysis techniques. The spectrum for the radical shows A, , at -335 nm in all three supports and is similar to that in solution. The radicals decay over time scales that vary from hundreds of nanoseconds to minutes and there are indications that some radicals may be decaying on shorter time scales than we can monitor. The efficiency of oxygen quenching increases in going from Silicalite to NaX to silica gel, consistent with the greater accessibility of oxygen to silica gel pores as compared to the narrow channels in Silicalite. Laser dose and ketone loading effects were also examined for the various supports. Potential applications of a kinetic treatment of the data based on dispersive reaction kinetics are also discussed as a means of dealing with the problem of decay kinetics that occur over a wide range of time scales. On a produit des radicaux diphknylmtthyles en excitant, l'aide d'un laser optrant a 266 nm, de la 1,1,3,3-tktraphknylacttone adsorbte sur du gel de silice ou incluse dans des ztolithes de NaX et de Silicalite et on les a ttudits en faisant appel aux techniques de photolyse <> a l'aide de laser i rtflectance diffuse. Le spectre du radical prksente un A, , a environ 335 nm dans chacun des trois supports et il est semblable a celui observt en solution. Les radicaux se dtcomposent a des vitesses qui varient de quelques centikmes de nanosecondes a des minutes; certaines indications nous laissent croire que certains radicaux pourraient se dkcomposer a des vitesses plus rapides que celles que l'on peut mesurer. L'efficacitt du pikgeage par l'oxygkne augmente lorsqu'on passe de la Silicalite au NaX au gel de silice; cet ordre est en accord avec la plus grande disponibilitk de l'oxygkne sur les pores du gel de silice par comparaison avec les canaux Ctroits qui existent sur la Silicalite. Pour divers supports, on a aussi examink les effets de la dose du laser ainsi que de la quantitC de cttone utiliste. On discute aussi des applications potentielles d'un traitement cinttique des donntes bast sur la cinttique de rkactions dispersives cornrne moyen de rksoudre le problkme de la cinktique de dtcompositions qui se produisent a des vitesses trks diffkrentes.

Following pulsed excitation at 354 nm, a transient absorption, maximizing at 510 nm, is readily observed for microcrystalline benzil, by diffuse reflectance laser flash photolysis. The transient absorption reduces the level of diffusely reflected analyzing light by 80%, and its decay appears to be a mixture of first- and second-order processes. Decay constants obtained from the slope of the first-order plot vary over a factor of about 2.5, with an initial slope = 0.18 × 105 (±0.01 × 10s) s−1, and a final slope = 0.66 × 104 (±0.04 × 104) s−1. Laser excitation also induces phosphorescence in microcrystalline benzil, which has a similar kinetic decay. It is confirmed that in diffuse reflectance mode the percentage transient absorption is directly proportional to the excited state concentration.

, 539 (1986).Transient absorption and luminescence from aromatic ketone triplets (xanthone, acetophenone, butyrophenone, valerophenone, p-methoxyacetophenone, P-phenyl-p-methoxypropiophenone and benzil) supported on the hydrophobic zeolite Silicalite have been obtained by time-resolved diffuse reflectance laser flash photolysis. These experiments were aimed at establishing the usefulness of this technique for studies of zeolite supported photoreactions. While the transient spectral properties can be used to examine the properties of the supporting media, the decays of the signals frequently follow complex kinetic behaviour. The decays for such heterogeneous systems cannot be easily fit to clean first or second order kinetics and are thought to be a mixture of several first order decays arising from triplets in different inclusion and adsorption sites. Evidence is presented for close-packing of the aromatic substrates and for triplet lifetime enhancement. Although the triplet spectrum of included benzil shows evidence for considerable axial freedom, the results for P-phenylpropiophenones indicate that other types of motion are restricted for included ketones. It is also found that the rate of the Norrish Type I1 reactions of valerophenone and butyrophenone supported on Silicalite are decreased with respect to the rates in solution. Faisant appel a la photolyse flash d'un laser a rCflexion diffuse rCsolue, on a mesure, l'absorption et la luminescence fugace d'itats triplets de cttones aromatiques (xanthone, acCtophCnone, butyrophCnone, valCrophCnone, p-mCthoxyacCtophCnone, P-phCnyl p-mCthoxypropiophCnone et benzil) supportCes sur le zeolite hydrophobe, Silicalite. On a rtalist ces expCriences dans le but d'ktablir 1'utilisC de cette technique pour des Ctudes de photorCactions supportCes sur des zeolites. Alors que l'on peut utiliser les propriCtCs spectrales fugaces pour examiner les propriCt6.s du support, les dCgCnCrescences des signaux permettent souvent de suivre des comportements cinCtiques complexes. Les dCgCnCrescences de tels systkmes hCtCrogknes ne suivent pas exactement des cinCtiques d'ordre un ou deux et on croit qu'elles sont des melanges de plusieurs dCgCnCrescences d'ordre un qui proviennent de triplets dans divers sites d'inclusion ou d'adsorption. On prCsente des donnCes suggCrant que les substrats aromatiques existent dans un arrangement compact et que les temps de vie des triplets sont augmentis. MCme si le spectre triplet du benzil prCsente beaucoup de libertC axiale, les rCsultats obtenus avec les (3-phCnylpropiophCnones indiquent que, pour ces cCtones, d'autres types de mouvements sont restreints. On a aussi trouvC que les vitesses de rCactions Norrish Type I1 de la valCrophCnone et de la butyrophCnone supportCes sur de la Silicalite sont plus faibles que les vitesses correspondantes en solution.[Traduit par le journal] Introduction Organic photoreactions on zeolite supports have been the subject of attention during the last couple of years (1-4). Our own studies have concentrated on the hydr...

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